Such a dehumidifier is already known from JP 60-198710. It has two identically constructed filter chambers that are each filled with a moisture-absorbing material. Air is drawn in from below through a bell arrangement with an oil sump and then passed through at least one of the two filter chambers where it passes through the moisture-absorbing material and is dried to leave the dehumidifier at its top. In addition an electrical heater is provided in each filter chamber. When the moisture-absorbing capacity of the moisture-absorbing material is reached in one of the filter chambers, the respective heater is turned on and the moisture-absorbing material is dried and regenerated. The driven-out moisture is vented from the filter chamber through an outlet.
In European patent application EP 1 313 112 a more developed dehumidifier is proposed. Downstream of a filter filled with moisture-absorbing material a dried-air stream passes over a humidity sensor that operates a heater inside the filter as needed. The humidity sensor determines the humidity of the air passing around it, which air is in theory already dried. When the humidity sensor detects a humidity level in this air stream that exceeds a predetermined limit, this indicates that the moisture-absorbing material is saturated, cannot take on any more water, and must be dried out. Then a switch closes the circuit for a heater, preferably a resistance heater, in the filter. As a result the moisture-absorbing material is heated and dried; the absorbed water drops down and out.
It has been shown that with the known dehumidifiers all of the cooked-out water neither leaves through the outlet above the heater nor at the lower vent. Instead, part of this driven-out water condenses as small droplets on the cylindrical inner wall of the filter chamber or housing of the dehumidifier. These small water droplets, depending on their surface tension, stay there and do not go away. This is undesirable since with time the humidity level at the inner surfaces constantly gets higher. As a result the readings of the described humidity sensor for the humidity of the air surrounding the drying medium does not accurately represent the condition of this medium.